1. Field of the Invention
The present invention relates generally to drive mechanisms and, more specifically, to a friction driven linear mass actuator capable of applying a controlled force to damp out oscillations in an attached structure or to excite oscillations in an attached structure.
2. Description of the Related Art
Vibration dampening can be achieved by controlling the movement of a mass to correspond to vibration induced movement of a corresponding structure. An example is disclosed in U.S. Pat. No. 4,643,385, issued to Sandercock discloses an active vibration dampening apparatus comprising an accelerometer, vibrating transducers, and isolating devices. The object is to actively cancel vibrations in the 2 to 25 Hz range. The output of a piezoelectric transducer is used to drive a speaker coil mounted in the base of the support structure. The control signal is out of phase with the input signal causing the cancellation of vibration. This device is particularly suitable for eliminating low frequency vibrations in sensitive equipment in a building. The mass being damped is necessarily small since the voice coil must support it.
U.S. Pat. No. 4,795,123, issued to Forward et al., discloses a vibrating object with three or more mutually perpendicular accelerometers and associated drivers. The driving actuators are attached to a vibrating structure in three planes and controlled by feedback of the accelerometers.
U.S. Pat. No. 4,796,823, issued to Schubert, describes an active vibration isolation system in which a system uses a velocity-sensitive geophone to sense payload velocity and modifies the signal to control the vibration-opposing force.
All of the aforementioned patents are severely limited in the amount of mass that may be used to reduce vibration because the masses are supported by flexible coils. Also, the devices are limited in frequency range so that they cannot be used to damp very low frequency vibration. Part of the reason for this is that the stroke of the mass for both of the devices described above is limited by having a coil structure.
In the field of aerospace technology, it is sometimes necessary to damp out vibrations in large space structures when they are deployed in the weightless environment of outer space. Also, for the purpose of testing these structures it is sometimes necessary to excite the structure on earth in a controlled test in order to study the dynamics thereof.
Previously used mass actuators for space applications have been driven by belts or gears using pneumatics or linear electric motors. Harris Corporation, for example, developed a linear actuator where magnets were affixed to the mass which passed through a linear motor. Beltac Corporation has developed a pneumatic belt driven actuator. However, the flexibility of a belt system prohibits accurate motion of the actuator due to the elasticity of the belt. The Harris design is also limited in terms of the amount of force that can be applied by the mass due to the type of motor used. Moreover, linear motors are susceptible to cogging effects which would severely limit their ability to function properly. A need exists for a mass actuator having a drive mechanism which is extremely accurate and capable of producing an offsetting oscillatory motions in response to sensed accelerations to actively damp out vibration and exciting oscillatory motions to test responses of structures in space and during ground testing on earth.